Thermostatic control system with smoke overload protection



Nov. 26, 1968 H. J. HElT 3,413,444

THERMOSTATIC CONTROL SYSTEM WITH SMOKE OVERLOAD PROTECTION Filed Nov. 12, 1965 2 Sheets-Sheet 1 BAKE BROlL MuLuoM sag 1 INVENTOR.

H ENRY I HEW ms ATTORNEY Nov. 26, 1968 H. J. HEIT 3,413,444

TEEPMUSTATIC CONTROL SYSTEM WITH SMOKE OVERLOAD PROTECTION Filed Nov. 12, 1965 2 Sheets-Sheet Z FIG.4

TEMR F" I OUEN AUR.

HRS. I Z

TEMP. F 8

o0 OUEU A\R ZOO- INVENTOR. HENRY 3. HEW

ms ATTORNEY United States Patent 3,413,444 THERMOSTATIC CONTROL SYSTEM WITH SMOKE OVERLOAD PROTECTION Henry J. Heit, Louisville, Ky., assignor to General Electric Company, a corporation of New York Filed Nov. 12, 1965, Ser. No. 507,415 Claims. (Cl. 219398) This invention relates to a temperature controlling system and particularly to apparatus for controlling the operation of a heating means in accordance with the sensed temperature of a body being heated, while providing smoke overload protection so as to restrict the rate of generation of smoke from the body under heavy soil conditions. i

This invention is a modification of the thermostatic control system described and claimed in my earlier application Ser. No. 369,719 which was filed on May 25, 1964, and entitled, Thermostatic Control System With Anticipation. This earlier application was also assigned to the General Electric Company, the assignee of the present invention and it issued as Patent No. 3,310,654 on Mar. 21, 1967.

This invention has found its greatest utility in controlling a high temperature baking and broiling oven where the oven is operated at both normal cooking temperatures between about 150 F. and 550 F. and at higher tem peratures between about 750 F. and 950 F. for degrading the food soil and grease spatter that accumulates on the walls of the oven liner while cooking is performed within the oven. One example of such a high temperature oven is a self-cleaning oven design taught by Bohdan Hurko in his Patent No. 3,121,158, which is also assigned to the General Electric Company.

Preferably a self-cleaning oven is provided with an oxidation unit in the oven exhaust outlet to promote the oxidation of carbon and carbon compounds so as to eliminate smoke, carbon monoxide, and other objectionable products exhausted from the oven cavity and returned to the kitchen atmosphere. A typical oxidation unit is disclosed by Stanley B. Welch in his Patent No. 2,900,483. Such an oxidation unit would normally include a source of heat such as an electric heating element and a. catalytic surface such as a platinum wire screen. As the oven exhaust gases pass through the oxidation unit, this heating element raises the exhaust gas temperature approximately 100 F. In the event of a heavy soil condition in the oven, secondary combustion begins to take place within the oxidation unit and the temperature within the unit increases rapidly. If the oven were relatively clean from the beginning, this combustion would not occur as no fuel would be supplied to the oxidation unit. In a soiled oven this secondary combustion in the oxidation unit may continue for about an hour until the dirty oven has been cleaned and the combustible fuel is depleted.

The present invention contemplates the control of the oven temperature during the high temperature self-cleaning cycle using a temperature sensor located on the output side of the oxidation unit so that the sensor would monitor not only the oven air temperature, but also the amount of combustible load in the oven. As combustion takes place in the oxidation unit, the sensor is heated up and causes the oven control to reduce the power supply to the oven heating means. This in turn slows the rate at which the combustible material in the oven is volatilized and therefore, the rate at which fuel is supplied to the oxidation unit. By the use of this invention it is possible to clean an oven having a heavy soil condition without overloading the oxidation unit and allowing objectionable smoke, odors and vapors to pass untreated into the kitchen atmosphere.

The principal object of the present invention is to pro- Patented Nov. 26, 1968 vide a high temperature oven with a temperature control means for accurately governing the energization of the heating means under different soil conditions so as not to overload a smoke eliminator or oxidation unit that is located in the oven exhaust outlet.

A further object of the present invention is to provide a self-cleaning oven having an oxidation unit located in the oven exhaust outlet with an improved temperature control system having a first temperature sensing means in heat transfer relation with the cooking compartment for controlling normal cooking temperatures, and an alternate temperature sensing means for use at self-cleaning temperatures above the cooking temperatures. This alternate sensing means is preferably located downstream of the oxidation unit so as to be able to detect combustion occurring in the oxidation unit and thereby cut back the power supplied to the heating means until this combustible material in the oven is volatilized so as to prevent the overloading of the oxidation unit and the passage of objectionable smoke into the kitchen atmosphere.

The present invention, in accordance with one form thereof, relates to a high temperature self-cleaning oven that is equipped with an oxidation unit for the oven exhaust outlet. The oven includes a temperature control means having a first temperature sensor in heat transfer relation with the cooking compartment for controlling the cooking temperatures between about F. and 550 F. A second temperature sensor is utilized in place of the first sensor at self-cleaning temperatures above the cooking temperatures. This second temperature sensor is located to monitor the oxidation unit exhaust temperatures so as to de-energize the heating means once secondary combustion takes place in the oxidation unit thereby retarding the rate of generation of combustible materials within the oven avoiding the overloading of the oxidation unit with smoke under heavy soil conditions.

My invention will be better understood from the following description taken in conjunction with the accompanying drawings and its scope will be pointed out in the appended claims.

FIGURE 1 is a left-side elevational view of a freestanding electric range with certain parts broken away and some in cross-section to show the main elements of an oven which is provided with the temperature control system embodying the present invention.

FIGURE 2 is a schematic wiring diagram of the power circuit in general and the temperature control circuits in particular for an oven heating system incorporating the present invention.

FIGURE 3 is a graph showing Time versus Temperature curves for both the oven air temperature and the oxidation unit exhaust air temperatures under both clean oven conditions and soiled oven conditions.

FIGURE 4 is a similar graph showing Time versus Temperature curves when using the invention embodied herein, wherein the oven air temperature is held down during the period when combustible materials are being volatilized so as to slow down the rate of volatilization or, in other words, the rate at which fuel is supplied to the oxidation unit.

Turning now to a consideration of the drawings and in particular to FIGURE 1, there is shown for illustrative purposes a free-standing electric range 10 having a top cooking surface or cooktop 11 with a plurality of surface heating elements 12, as well as an oven cavity 13 formed by a box-like oven liner 14 and a front-opening drop door 15. The oven cavity 13 is supplied with the usual source of heat energy; namely, two electric resistance heating elements in the form of a lower baking element 16 and an upper broiling element 17 although other heating means could be substituted. A third heating element has been added adjacent the oven door 15, and it is a mullion or perimeter heater 18 that is wrapped around the oven liner in a manner that is taught by James K. Newell in his Patent No. 3,017,488 which is also assigned to the General Electric Company.

The control of the surface heating elements 12 is obtained by a series of selector switches 20 which are illustrated as multiple pushbutton switches arranged in pairs along the opposite side arms of the cooktop 11 to be oriented with the particular heating element that is being controlled thereby. In addition, the range is provided'with a backsplash 21 that is arranged along the back edge of the cooktop and is provided with a control panel 22 which contains most of the remaining controls for the surface heating elements 12 and for the oven compartment.

Other structural features that might be mentioned in passing would be the outer range body or cabinet 27 which has an enameled or metallic appearance finish which supports and encloses the various components of the range. Sandwiched between the oven liner 14 and the range body 27 is a thick blanket of insulating material 28 such as fiber glass or the like. The amount of insulation being used has been increased over the amount of insulation for a standard oven because of the high operating temperatures encountered during the heat cleaning operation which will reach a maximum oven air temperature somewhere between about 750 F. and 950 F.

During the heat cleaning operation free carbon, soot and carbon monoxide gases are formed and these must be oxidized before returning them to the kitchen atmosphere so as to prevent air contamination. A catalytic oxidation unit 30 is positioned in an oven vent opening 31 to serve as an exhaust means for the oven. Such an oxidation unit includes a heating means in cooperation with a catalytic platinum surface such as a wire screen that is heated to a temperature of about 1300 F. and tends to consume the smoke, odors and vapors emanating from the oven cavity as was mentioned previously.

It has been found desirable to provide a latch means for the oven door 15 to insure that the oven door is closed and cannot be opened when the oven is operated on a heat cleaning cycle. The particular door latching mechanism is identified generally as element 34 in EIG- URE l, althought it is best disclosed in the patent of Clarence Getman No. 3,189,375, which is also assigned to the General Electric Company.

Turning now to a consideration of the oven circuit diagram of FIGURE 2, there is provided an electrical service of three-wire Edison source of power; nominally of 240 volts, single phase, 60 cycle AC which is usually available in the average residence having adequate wiring. This voltage source has a pair of line wires L1 and L2 and a grounded neutral conductor N for supplying energy to the electrical load of the oven made up of the bake element 16, the broil element 17 and the mullion heater 18. This power circuit is controlled by an oven selector switch 36 which may be either a rotary switch or a multiple pushbutton switch interposed between the voltage source and the load, as is conventional in this art. A switch 37 is shown in line L1 and it is governed by an oven timer (not shown) in order to obtain timed baking operations.

The three types of cooking operations to be performed in the oven are Baking, Broiling and Timed Baking. A typical heating circuit for the Baking operation would be to have the bake element 16 connected to a source of high voltage across line wires L1 and L2 at 240 volts, while at the same time the broil element 17 and the mullion heater 18 are series connected across lines L1 and L2 at 240 volts. A circuit for a typical broiling operation has only the broiler element 17 energized, and it is at 240 volts across lines L1 and L2. The Timed Baking operation has the same circuit as the Baking operation except the circuit is controlled by the timer switch 37 as is well understood in this art. The normal cooking temperature range is from about 150 F. to about 550 F. The Broiling operation has the highest cooking temperature setting of about 550 F.

It is necessary to provide an oven temperature control system for governing the power supply to heating elements so as to reach and hold set temperature levels which is necessary for proper cooking results. Such a temperature control system is respresented by an electrical control circuit network which forms the subject matter of the present invention, and it is connected in the power circuit for cycling the heating elements and maintaining a stable condition. The network is operated at a low-voltage of about 12 volts from the secondary of a step-down transformer 39 whose primary is connected by leads 40 and 41 across line L1 and neutral N through the oven selector switch 36.

The principal components of the circuit are first a variable-resistance temperature sensor 43 that is adapted to be located in heat transfer relationship with the oven cavity 13 (shown at the rear, top corner of the oven liner in FIGURE 1) and second, a responder comprising the combination of a potentiometer 45, a thermally responsive member in the form of a U-shaped bimetal or polymetallic member 46 having supported on one leg 47 a responder winding 48 in series with the potentiometer 45. An anticipator winding 49 is supported around the other leg 50 of the bimetal 46. Lastly, a voltage regulating winding 51 is located adjacent the anticipator winding 49 on the leg 50 of the bimetal and is connected across the secondary of the transformer 39 by means of leads. One end of the bimetal is fixed as at 52, While the opposite end supports a movable contact 53 that cooperates with a fixed contact 54 and thereby forms a responder switch. The anticipator winding 49 cooperates with the responder winding 48 so as to reduce the amplitude of the temperature changes that are required to turn the temperature control On and Off. With no form of anticipation, the temperature change required to effect an operation of the control would be very large. This anticipator winding 49 is connected to the responder contact 56 by means of lead 57, and the opposite end of the anticipator winding is joined by lead 58 to a hot wire relay winding 59 which is the output relay of the responder and includes relay switch 60 which is interposed in the power circuit. The opposite side of the hot wire relay winding 59 is connected back to the secondary of the transformer 39 by means of lead 61. Moreover, the responder bimetal 46 is joined back to the other side of the transformer secondary by means of lead 52.

As mentioned previously, the oxidation unit 30 has a heater winding identified in the wiring diagram of FIG- URE 2 as heater that is connected across line L1 and neutral N by leads 41 and 66. The present invention contemplates the substitution of a second temperature sensor cooperating with the oxidation unit 30 during the high temperature heat cleaning cycle in place of the first temperature sensor 43. This exchange of temperature sensors is made possible by a single pole double throw switch 71 having switch contacts 72 and 73. This switch 71 is interposed between the responder winding 48 and the two temperature sensors 43 and 70 so that during normal cooking operations the switch is closed with contact 72 so as to make a series connection between the first temperature sensor 43, the potentiometer 45 and the responder winding 48 across the secondary of the transformer by means of leads 75. and 52 respectively. When it is desirable to initiate the high temperature heat cleaning cycle, the switch 71 is thrown to open the circuit through switch contact 72 and close with switch contact 73 thereby cutting out the first temperature sensor 43 and the potentiometer 45 and connecting the second temperature sensor 70 in series with the responder winding 48.

An additional method of providing anticipation is added to the circuit during the high temperature cycle in the form of a resistor 76, separate from the bimetal. This resistor 76 has one terminal connected by lead 77 between the sensor 70 and switch contact 73, while its other terminal is connected by lead 78 to a connection between the hot wire relay 59 and anticipator winding 49 by means of leads 79 and 58 respectively. When the oven is up to temperature, that is, no additional heat is demanded, the responder contacts 55 and 56 are opened. This forms a series circuit with the resistor 76 and the relay 59 to be in parallel with the sensor 70. The effect of this is to increase the voltage drop across the responder winding 48 and this increases the force tending to close the responder contacts 55 and 56. When the temperature of the oven drops below the preset temperature the responder contacts 55 and 56 will close again, thus forming a series circuit with resistor 76 and the anticipator winding 49 in parallel with the responder winding 48. The effect of this is to decrease the voltage drop across the responder winding 48 and this increases the force tending to open the responder switch contacts 55 and 56 ahead of the time they would normally open. This resistor 76 provides additional anticipation for the high temperature range of the heat cleaning cycle, but it is not used in the control circuit during normal cooking operations.

The location of the second temperature sensor 70 is critical so that it will sense and monitor the temperatures of the exhaust gases leaving the oxidation unit 30. Thus, the second temperature sensor 70 is located downstream of the oxidation unit 30 as is shown in dotted line in FIGURE 1.

For a better understanding of the present invention, attention is directed to the graph of FIGURE 3 of the drawing where the abscissa denotes Time measured in hours and the ordinate is marked in Temperature measured in degrees Fahrenheit. If there were little or no soil present in the cooking compartment 13, the oven air temperature curve A would rise rapidly until it reaches a maximum temperature of about 850 F. during a time interval of about one-half an hour where it will stabilize for a desired period which would be governed by an oven timer (not shown).

The oxidation unit 30 has a heater winding 65 of about 160 watts located therein as a source of additional heat for the catalytic screen (not shown) present therein. As the oven exhaust gases enter the vent opening 31 and pass through the oxidation unit, this heater 65 raises the exhaust temperature about 100 F. This oxidation unit output air temperature is shown in the temperature curve B in the graph of FIGURE 3, and you will notice that it follows very closely to the oven air temperature curve A. It must be remembered, however, that this condition prevails in a substantially clean oven only.

In the event a heavy soil condition exists in the cooking compartment 13, the oxidation unit output air temperature follows generally a temperature curve C. The two temperature curves A and C approximate each other for about the first fifteen minutes. When the oxidation unit reaches a temperature of about 650 F. combustion begins to take place inside the unit and the oxidation unit temperature increases rapidly. This is true because fuel in the form of partially volatilized food soil, is being supplied to the oxidation unit, and this condition prevails as long as combustible materials are volatilized in the cooking compartment 13. Notice that the maximum temperature of the oxidation unit output air reaches in the neighborhood of 1300 F. This condition prevails until the soil has been completely eliminated at which point the oxidation unit exhaust temperature drops until the two temperature curves A and C are again very similar.

The invention contemplated herein substitutes a second temperature sensor 70 in place of the normal temperature sensor 43 during the high temperature cycle, and this second sensor is located on the output side or downstream of the oxidation unit 30. In this position, the sec- 0nd temperature sensor 70 would sense not only the oven air temperature, but also the amount of combustible load in the oven. For a clean oven, the temperature curve of the oxidation unit would be curve B. However, for a dirty oven, the temperature curves are best understood by referring to a second graph of FIGURE 4. In this graph, the oven air temperature curve is identified as curve D, while the oxidation unit output air temperature curve is identified as curve E. Looking first at curve E, when combustion begins to take place in the oxidation unit at a temperature between 500' F. and 600 F., this temperature curve rises rapidly and the second temperature sensor 70 will become heated up and will cause the responder contacts 55 and 56 to open thereby de-energizing the hot wire relay 59 and hence the power circuit to the heating means 16, 17 and 18. When this occurs the oven air temperature curve will fall oif as is shown at point 76. Since the oven air temperature curve D remains below about 700 F. until the combustible materials in the oven are volatilized, the oxidation unit output air temperature curve E is likewise lowered until both the curves D and E assume normal temperature having about a F. differential. By using this invention the amount of soil loading in the oven may be greatly increased without overloading the oxidation unit, for when the oxidation unit is overloaded it does not perform its function, and smoke odors and vapors are returned to the kitchen atmosphere without proper treatment.

Modifications of this invention will occur to those skilled in this art therefore it is to be understood that this invention is not limited to the particular embodiments disclosed, but that it is intended to cover all modifications which are within the true spirit and scope of this invention as claimed.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An oven comprising walls defining a cooking compartment, heating means for said compartment for establishing cooking operations as well as a high temperature self-cleaning operation, control means for said heat ing means selectively operable to control the heating means to effect either a cooking operation or a self-cleaning operation, temperature control means for said heating means settable to control the heating means when in a cooking operation to hold selected temperatures wihin a temperature range between about F. and 550 F. and within a self-cleaning temperature range between about 750 F. and 950 F., and an oven exhaust outlet in one wall of the housing, an oxidation unit mounted in the said exhaust outlet, said temperature control means including a first temperature sensor located within the cooking compartment for detecting the oven air temperature during the cooking operation, while said temperature control means includes a second temperature sensor located downstream of the oxidation unit for alternately detecting oven exhaust temperatures during the self cleaning operation so as to prevent the oxidation unit from becoming overloaded while the oven is being cleaned of a heavy soil load.

2. A high temperature self-cleaning oven having walls defining a cooking compartment, heating means for said compartment, control means for selectably operating said heating means, temperature control means for governing said heating means to have a maximum compartment temperature of about 950 F., and an oven exhaust outlet in one of the compartment walls, an oxidation unit mounted in said exhaust outlet, said temperature control means including a temperature sensor located downstream of the oxidation unit remote from said cooking compartment for monitoring the oven exhaust temperatures and de-energizing the heating means at exhaust outlet temperatures above about 1050 F. when the oxidation unit becomes overloaded due to a heavy soil condition in the oven compartment.

3. A high temperature self-cleaning oven having walls defining a cooking compartment, heating means for said compartment, control means for selectably operating said heating means, temperature control means for governing said heating means, and an oven exhaust outlet in one of the compartment walls, and a catalytic oxidation unit located in the exhaust outlet, said temperature control means having a first temperature sensor in heat transfer relation with the cooking compartment for controlling the cooking temperaures between about 150 F. and 550 F., said temperature control means including a second temperature sensor for use at self-cleaning temperatures above the cooking temperatures, said second temperature sensor being located to monitor the oxidation unit exhaust temperatures at a maximum temperature between about 850 F. and 1050 F. so as to tie-energize the heating means once the exhaust temperatures reach the critical range thereby retarding the rate of heating the compartment and hence the rate of smoke generation so as to avoid overloading the oxidation unit due to heavy soil conditions.

4. A domestic oven having insulated walls defining a cooking cavity, one wall having an opening that is provided with a door for gaining access thereto, an oven exhaust opening formed in one wall of the oven cavity, and an oxidation unit combined with the exhaust opening, heating means for said cooking cavity, control means for said heating means selectively operable to attain various rates and directions of heating, and a temperature control means for said heating means selectably operable to hold a predetermined maximum temperature within the cooking cavity, said temperature control means comprising a first temperature sensor in heat transfer relationship with the cooking cavity for use in a cooking temperature range between about 150 F. and 550 F., a selectably operable temperature responder subject to the action of the first temperature sensor, and a relay means governed by the responder means for controlling the energization of the heating means, a second temperature sensor for alternate use in place of the first temperature sensor, said second temperature sensor being located to monitor the oxidation unit exhaust temperatures at a critical maximum temperature between about 850 F. and 1050 F. so as to avoid overloading the oxidation unit due to heavy soil conditions in the oven.

5. In a domestic oven as recited in claim 4 wherein the temperature control means includes a circuit network having a source of voltage with means for regulating the voltage so that it is independent of variations in the voltage source, the first temperature sensor being a variable-resistance located within the cooking cavity and in References Cited UNITED STATES PATENTS 2,344,770 3/1944 Gunness 23255 X 3,082,311 3/1963 Chisholm 219413 3,121,158 2/1964 Hurko 219397 3,150,619 9/1964- Bruchen et al. +8 3,270,183 8/1966 Jordan 219398 3,273,971 9/1966 Baddorf et al. 23288 3,290,483 12/1966 Hurko 219--393 BERNARD A. GILHEANY, Primary Examiner.

VOLODYMYR Y. MAYEWSKY, Assistant Examiner. 

1. AN OVEN COMPRISING WALLS DEFINING A COOKING COMPARTMENT, HEATING MEANS FOR SAID COMPARTMENT FOR ESTABLISHING COOKING OPERATIONS AS WELL AS A HIGH TEMPERATURE SELF-CLEANING OPERATION, CONTROL MEANS FOR SAID HEATING MEANS SELECTIVELY OPERABLE TO CONTROL THE HEATING MEANS TO EFFECT EITHER A COOKING OPERATION OR A SELF-CLEANING OPERATION, TEMPERATURE CONTROL MEANS FOR SAID HEATING MEANS SETTABLE TO CONTROL THE HEATING MEANS WHEN IN A COOKING OPERATION TO HOLD SELECTED TEMPERATURES WITHIN A TEMPERATURE RANGE BETWEEN ABOUT 150*F. AND 500*F. AND WITHIN A SELF-CLEANING TEMPERATURE RANGE BETWEEN ABOUT 750*F. AND 950*F., AND AN OVEN EXHAUST OUTLET IN ONE WALL OF THE HOUSING, AND OXIDATION UNIT MOUNTED IN THE SAID EXHAUST OUTLET, SAID TEMPERATURE CONTROL MEANS INCLUDING A FIRST TEMPERATURE SENSOR LOCATED WITHIN THE COOKING COMPARTMENT FOR DETECTING THE OVEN AIR TEMPERATURE DURING THE COOKING OPERATION, WHILE SAID TEMPERATURE CONTROL MEANS INCLUDES A SECOND TEMPERATURE SENSOR LOCATED DOWNSTREAM OF THE OXIDATION UNIT FOR ALTERNATELY DETECTING OVEN EXHAUST TEMPERATURES DURING THE SELF CLEANING OPERATION SO AS TO PREVENT THE OXIDATION UNIT FROM BECOMING OVERLOADED WHILE THE OVEN IS BEING CLEANED OF A HEAVY SOIL LOAD. 